"There is the possibility of further copy-cat attacks. All Defence personnel should [avoid] known or likely trouble spots and take extra care in what you reveal about yourself and your planned activities in social media," he said.

Case raised in Question Time

The shockwaves have spread to Canberra, with Prime Minister Julia Gillard commenting on Al-Ahmadzai's arrest during Question Time.

"Agents of the Australian Federal Police and the NSW Police Force last night arrested a 23-year-old man in Sydney," she said.

"The man is charged with using a carriage service to make a threat and threatening to cause harm to a Commonwealth official.

"He has been refused bail. Speaker, I do not intend to comment on the specifics of this case as it is before the courts."

Just an hour earlier, all details of the case were made secret for national security reasons.

Late on Tuesday the order was lifted, but the Opposition is demanding answers over why Ms Gillard mentioned the case at all.

"Can you advise the House whether she has by her actions prejudiced the trial of the accused?" asked Manager for Opposition Business Christopher Pyne.

But Attorney-General Mark Dreyfus says Ms Gillard did not breach the suppression order.

"The suppression order was not only not brought to our attention but the New South Wales Police, [the] website was showing the information that the Prime Minister used in her answer," he said.